Reducing the environmental impact of coatings compositions, in particular that associated with emissions into the air of volatile organics during their use, has been an area of ongoing investigation and development in recent years. Accordingly, interest in powder coatings has been increasing due, in part, to their inherently low volatile organic content (VOC), which significantly reduces air emissions during the application process. While both thermoplastic and thermoset powder coatings compositions are commercially available, thermoset powder coatings are typically more desirable because of their superior physical properties, e.g., hardness and solvent resistance.
Low VOC coatings are particularly desirable in the automotive original equipment manufacture (OEM) market, due to the relatively large volume of coatings that are used. However, in addition to the requirement of low VOC levels, automotive manufacturers have very strict performance requirements of the coatings that are used. For example, automotive OEM clear top coats are typically required to have a combination of good exterior durability, acid etch and water spot resistance, and excellent gloss and appearance. While liquid top coats, in particular epoxy-acid cured liquid coatings, can provide such properties, they have the undesirable drawback of higher VOC levels relative to powder coatings, which have essentially zero VOC levels.
Epoxy based powder coatings, such as epoxy-acid powder coatings, are known and have been developed for use as automotive OEM clear top coats. However, their use has been limited due to deficiencies in, for example, flow, appearance and storage stability. Epoxy based powder coating compositions typically comprise a co-reactant, e.g., a crosslinking agent, having functional groups that are reactive with epoxides, e.g., dodecanedioic acid, and an epoxy functional polymer, e.g., an acrylic copolymer prepared in part from glycidyl methacrylate. The epoxy functional polymers used in such epoxy based powder coatings compositions are typically prepared by standard, i.e., non-living, radical polymerization methods, which provide little control over molecular weight, molecular weight distribution and polymer chain structure.
The physical properties, e.g., glass transition temperature (Tg) and melt viscosity, of a given polymer can be directly related to its molecular weight. Higher molecular weights are typically associated with, for example, higher Tg values and melt viscosities. The physical properties of a polymer having a broad molecular weight distribution, e.g., having a polydispersity index (PDI) in excess of 2.0 or 2.5, can be characterized as an average of the individual physical properties of and indeterminate interactions between the various polymeric species that comprise it. As such, the physical properties of polymers having broad molecular weight distributions can be variable and hard to control.
The polymer chain structure, or architecture, of a copolymer can be described as the sequence of monomer residues along the polymer backbone or chain. For example, an epoxy functional copolymer prepared by standard radical polymerization techniques will contain a mixture of polymer molecules having varying individual epoxy equivalent weights. Some of these polymer molecules can actually be free of epoxy functionality. In a thermosetting composition, the formation of a three dimensional crosslinked network is dependent upon the functional equivalent weight as well as the architecture of the individual polymer molecules that comprise it. Polymer molecules having little or no reactive functionality (or having functional groups that are unlikely to participate in crosslinking reactions due to their location along the polymer chain) will contribute little or nothing to the formation of the three dimensional crosslink network, resulting in less than optimum physical properties of the finally formed polymerizate, e.g., a cured or thermoset coating.
The continued development of new and improved epoxy based powder coatings compositions having essentially zero VOC levels and a combination of favorable performance properties is desirable. In particular, it would be desirable to develop epoxy based powder coatings compositions that comprise epoxy functional polymers having well defined molecular weights and polymer chain structure, and narrow molecular weight distributions, e.g., PDI values less than 2.5. Controlling the epoxy polymer architecture and polydispersity is desirable in that it enables one to achieve higher Tg's and lower melt viscosities than comparable epoxy polymers prepared by conventional processes, resulting in thermosetting particulate compositions which are resistant to caking and have improved physical properties.
International patent publication WO 97/18247 and U.S. Pat. Nos. 5,763,548 and 5,789,487 describe a radical polymerization process referred to as atom transfer radical polymerization (ATRP). The ATRP process is described as being a living radical polymerization that results in the formation of (co)polymers having predictable molecular weight and molecular weight distribution. The ATRP process is also described as providing highly uniform products having controlled structure (i.e., controllable topology, composition, etc.). The '548 and '487 patents and WO 97/18247 patent publication also describe (co)polymers prepared by ATRP, which are useful in a wide variety of applications, for example, with paints and coatings.